1. Technical Field
This invention relates to afocal zoom lens systems and, more particularly, to such lens systems which are capable of operating in the infrared spectrum.
2. Description of Related Art
There are many applications for optical lens systems capable of operating in the infrared spectrum. For purposes of this invention, the term "infrared" means electromagnetic radiation having a wavelength longer than visible radiation and shorter than microwave radiation. The numerical wavelength range of the infrared spectrum is usually considered to extend from 0.7 microns which is the longest visible wavelength to substantially 100 microns. Infrared lens systems can be used as part of a night viewing surveillance device, an aircraft navigation device used to provide all-weather and night flying capabilities, as well as known infrared radar and imaging systems. One such system is generally referred to by the acronym "FLIR" derived from the words "forward looking infrared". These systems are preferably operated in the 2-20 micron wavelength region of the spectrum, more particularly to the 8-12 micron range.
It is often desirable to provide a zoom lens system that is capable of operating in the infrared spectrum. Such a zoom system can be used as a "bolt on" attachment to a primary imaging system such as that encountered in conventional FLIR systems. Typically, the zoom system must be of the afocal type wherein the output of the zoom is a collimated beam of energy focused at infinity. Among the desirable design parameters is that the afocal infrared zoom provides small transmission losses while at the same time providing usefully high image quality. Transmission losses are particularly acute with lenses having a spectral bandpass in the infrared region. Consquently, both from a cost and performance standpoint the number of lenses must be kept to a minimum. In addition, a compact design is also highly desirable.
Most of the previously known zoom systems, even in the visible wavelength region, are of the finite focus or image forming output type rather than of the afocal or attachment type intended for use with a prime focusing lens. U.S. Patents typical of this image forming type at visible wavelengths include U.S. Pat. No. 3,377,119 to Takano; U.S. Pat. No. 3,433,559 to Vockenhuber et al; U.S. Pat. No. 3,454,321 to Klein; and U.S. Pat. No. 3,597,048 to Bertele. Focusing lens systems intended for use at infrared wavelengths are shown in U.S. Pat. Nos. 3,439,969 to Kirkpatrick and 3,825,315 to Altman et al.
In more recent years the increasing use of far-infrared (8-12 microns) scanning systems have led to more development in the area of infrared afocal telescopes with variable magnification, i.e., zoom lens systems. Variable magnification or zoom telescopes have been designed and manufactured, but most have been limited in use due to their excessive physical size. Other constraints on the use of such lenses have been their mechanical complexity and difficulty in attaining both their theoretical optical design performance and their required mechanical reliability.
An excellent review of infrared zoom lens development is found in Neal, "Zoom Lenses for the Thermal Infrared", Optical Systems Design, Analysis, and Production; Volume 339, April of 1983. In that paper, the author discusses a design disclosed in U.S. Pat. No. 3,947,084 to Noyes that provided a 6:1 zoom ratio but unfortunately was not of a particularly short overall length. This zoom system utilizes a mechanically compensated zoom objective system and a two element eyepiece system between which an internal real image is produced. The zoom objective system which contains two separate nonlinear zooming components has the disadvantage of requiring the zoom component which is adjacent to the image to travel close to the real image. The zoom construction disclosed in this patent employed a re-imaging afocal zoom construction as compared to a Galilean type where the real image is not formed internally of the lens system.
Later, a more compact telescope design which provided a 4:1 zoom ratio was described in Roberts, "Compact Infrared Continuous Zoom Telescope", Optical Engineering, Vol. 23, No. 2, March/April 1984. Disclosed therein is a telescope formed by a mechanically compensated zoom objective system and a three-element eyepiece system. Although this design provided an improvement over previous zoom telescopes, the overall length required further shortening and apart from there being only a medium sized zoom range, the low magnification did not offer a particularly wide field of view. Other publications referring generally to the infared zoom technological area include Jamieson, "Zoom Lenses for the 8u-13u Waveband", Optica Acta, Vol. 18, No. 1, (1971).
In addition to the design objectives noted above there exists a need for an infrared zoom lens system capable of providing continuous-in focus view of a scene (i.e., afocal type), with the zoom lens being capable of a relatively large magnification ratio of at least 8:1 while also providing the capability of providing a unit magnification power (1.0X) mode of operation while at the same time providing a compact structure. The capability to provide a unit power zoom position is important for navigation applications which require the displayed scene to correspond exactly to the actual scene the pilot would see with direct viewing. The unit power zoom position would provide the pilot with the exact field of view necessary to fly his aircraft. Still other applications for an infrared afocal zoom lens system with these capabilities will become apparent to those skilled in the art.